(1) Field of the Invention
The present invention relates to a retort-sterilizable laminated structure and a process for the preparation thereof. More particularly, the present invention relates to a retort-sterilizable sealing and packaging material excellent in the flavor retaining property and the interlaminar peel strength after retort sterilization.
Furthermore, the present invention relates to an improvement in the process for the preparation of laminated structures comprising heat-bonding an aluminum foil to a heat-sealable olefin resin layer through a modified olefin resin layer formed by melt extrusion.
(2) Description of the Prior Art
As a retort-sterilizable sealing and packaging material, there have heretofore been broadly used laminated sheets comprising an aluminum foil or sheet, a heat-sealable crystalline olefin resin layer formed on one surface of the aluminum foil or sheet and a heat-resistant resin layer, such as a polyethylene terephthalate layer, formed on the other surface of the aluminum foil or sheet. Such laminated sheet is piled and the periphery is heat-sealed to form a bag or pouch, and food or other content is packed in the bag or pouch and after degasification, the bag or pouch is sealed. Then, the sealed bag or pouch is heated and sterilized in a sterilizing apparatus called "retort" to obtain a packaged food or the like which can be preserved and stored under normal conditions.
A crystalline olefin resin used as a sealing material and an inner face material is ordinarily inactive, and a packaging material such as mentioned above is subjected to the heat-sterilizing treatment conducted under severe conditions. Accordingly, bonding of an aluminum foil or sheet to such olefin resin layer is mainly performed by using a thermosetting adhesive such as a urethane type adhesive (isocyanate type adhesive) or an epoxy type adhesive.
Such thermosetting adhesive is ordinarily satisfactory for attaining an object of providing a bonding excellent in the heat resistance and the resistance to hot water, but it is still insufficient in the property of retaining the flavor of packed food after retort sterilization.
A thermosetting resin is a complicated resinous composition comprising uncondensed monomers and polycondensates having a relatively low molecular weight. Furthermore, in the above-mentioned packaging material, the thermosetting adhesive layer is always present inside an impermeable metallic aluminum layer. Accordingly, when the packaging material is subjected to a severe treatment such as retort sterilization, the above-mentioned uncondensed monomers or oligomer tend to migrate in or react with the packed food. The above-mentioned thermosetting adhesive is ordinarily used in the form of a dilution with an organic solvent such as toluene, ethyl acetate or methylethyl ketone. Accordingly, when an aluminum foil is laminated on a crystalline olefin resin film by using such thermosetting adhesive, there should be adopted the steps of coating the adhesive on the aluminum foil, drying the coated aluminum foil by a hot air oven to evaporate the organic solvent and press-bonding the crystalline olefin resin film to the adhesive-coated aluminum foil to form a laminated structure. However, it is very difficult to remove the organic solvent completely by oven drying, and the residual organic solvent is caused to migrate into the packed food on retort sterilization and the flavor of the packed food is drastically degraded in some case. Thus, a packaging material composed of a laminated sheet comprising a crystalline olefin resin layer and an aluminum foil or sheet bonded thereto by a thermosetting adhesive is not satisfactorily in the property of retaining a packed food, though this packaging material is excellent in the food preserving property.
As the thermoplastic resin adhesive for heat-bonding a polyolefin layer to a metal substrate, there is known an olefin resin modified with an unsaturated carboxylic acid or the like. However, it has been found that when a modified propylene polymer is used for heat-bonding a crystalline propylene polymer to an aluminum foil or other metal substrate, various disadvantages are caused. For example, when an aluminum foil or the like is heat-bonded to a crystalline propylene polymer layer, it is necessary to apply a modified propylene resin between the two layers. However, modified propylene resins customarily used in this field are very brittle and it is very difficult to extrude them in the form of uniform and thin films. Therefore, in the conventional process for preparing laminated structures by using a modified propylene resin, there must be adopted complicated and troublesome steps of forming a dispersion of a modified propylene resin, coating the dispersion on an aluminum foil, removing the solvent by drying and heat-bonding the aluminum foil to a crystalline polypropylene layer.
Moreover, the heat-bondability of this modified propylene resin at a low temperature is very poor, and hence, the heat-bonding operation should be conducted at a high temperature. When a laminate is prepared at such high temperature, wrinkles are readily formed on the resulting laminate.
Furthermore, a laminated structure prepared according to the above procedures is defective in that when it is subjected to retort sterilization, interlaminar peeling is readily caused under shocks.